1. Field of the Invention
The present invention relates to a color image display apparatus which displays a color video image by controlling light emission of red (R), green (G) and blue (B) primary colors, and more particularly, to a color image display apparatus with an excellent dynamic resolution characteristic, which displays a high-quality moving image where color fringes at moving image edges are inconspicuous.
2. Description of the Prior Art
In recent years, in place of conventional Braun tube (CRT) display devices, flat-panel type display devices are becoming popular. These thin and light display panel devices, having a display panel where liquid crystal or plasma is sealed, display images with reduced image distortion, and receive reduced influence of earth magnetism. Among the flat-panel display devices, a plasma display device particularly draws public attention as a next-generation color image display device. The plasma display device is a spontaneous light emitting device, and therefore it has a wide view angle. Further, a large panel can be relatively easily constructed for this device. In this flat-panel display device, one pixel consists of red (R), green (G) and blue (B) light emitting cells. Color image display is realized by controlling the light emitting luminance levels of the respective light emitting cells.
Further, the plasma display device or the like having difficulty in displaying gray scale representation between xe2x80x9clight emission (turned on)xe2x80x9d and xe2x80x9cnon light emission (turned off)xe2x80x9d, employs a so-called subfield method for displaying the gray scale representation by controlling the light emitting luminance levels of the respective R, G and B light emitting cells. In the subfield method, one field is divided into a plurality of subfields on a time base, then light emitting weights are uniquely allotted to the respective subfields, and light emission in the respective subfields are on/off controlled. This attains luminance gradation (or tonality) representation.
For example, in a case where one field is divided into six subfields SF0 to SF5 and light emitting weights in the ratios 1:2:4:8:16:32 are respectively allotted to the subfields, 64 level gradation can be represented. At level xe2x80x9c0xe2x80x9d, light emission is not performed in any of the subfields SF0 to SF5. At level xe2x80x9c63xe2x80x9d (=1+2+4+8+16+32), light emission is performed in all the six subfields.
In this manner, in the color image display device which controls the light emitting luminance levels of respective R, G and B light emitting cells by the subfield method, the image quality of a displayed moving image is greatly influenced by time response characteristics related to light emission by the R, G and B cells (hereinafter may be simply referred to xe2x80x9clight emitting response characteristicsxe2x80x9d) and the array of light emitting weights allotted to the respective subfields in each field.
The light emitting response characteristics of the R, G and B cells respectively indicate a light-emitting rise time characteristic from a point where a controller has instructed to start light emission to a point where light emitting luminance at the cell actually reaches a desired level, and a persistence time characteristic after the light emission instruction. Generally, if the persistence time is long, the light-emitting rise time is long. Accordingly, the persistence time is used as a representative characteristic of light emitting response characteristic. In the following description, the light emitting response characteristic is represented by the xe2x80x9cpersistence timexe2x80x9d (a period from a point where the light emission is at the peak to a point where the light emission is at a level {fraction (1/10)} of the peak). The xe2x80x9cpersistence timexe2x80x9d includes the xe2x80x9clight-emitting rise time characteristicxe2x80x9d.
The operation of this color image display device can be ideal operation as the light emitting response characteristics are short, however, the light emitting response characteristics cannot be reduced to zero. Further, as the light emitting response characteristics greatly depend on physical characteristics such as fluorescent materials used as the light emitting cells, it is very difficult to obtain uniform response characteristics in the R, G and B cells having different luminous wavelengths. For these reasons, when a moving image is displayed, the differences in time responses of the respective light emitting cells cause time lags in R, G and B light emission which overlap with each other, resulting in color shift (color fringing). The color shift appears at an edge portion where luminance greatly changes, e.g., from black to white or vice versa, as a phenomenon that a color different from the original image color is perceived. This seriously degrades image quality in moving image display.
Hereinbelow, the process of occurrence of color fringing interference at edge portions will be described with reference to FIG. 3 and FIGS. 4A and 4B. As shown in FIG. 3, a white rectangular pattern 32 on black background 31 is displayed on a display screen of a display device, and the white rectangular pattern 32 is moved rightward in FIG. 3. FIGS. 4A and 4B show color fringes occurred on the boundaries between white and black colors.
FIG. 4A shows the intensities (amplitudes) in the respective light emitting cells. FIG. 4B shows colors displayed on the display screen. As shown in FIG. 4A, as the G light emitting response is slower than the R and B light emitting responses, the G light emitting response represented with the broken line is delayed from the R and B light emitting responses represented with the solid lines. Thus, color fringing occurs in edge areas A and B. As shown in FIG. 4B, in the edge area A, a color of magenta (R+B) is perceived due to shortage of the amplitude of G with respect to R and B. In the edge area B, a color of green (G) is perceived due to excess amplitude of G. The edge area where color fringing occurs becomes wider as the speed of moving image increases.
In this manner, in the white and black video signal, colors not included in the original image (magenta and green) are perceived depending on the motion of the image. This seriously degrades the image quality. Especially, in the plasma display device and the like, material having persistence time of 12 ms or longer is often used as a G light emitting cell. As the response of the G cell using this material is slower than the responses of R and B cells, the consequent color fringing in edge areas is a main factor of degradation of image quality.
On the other hand, in the display devices which displays gray scale representation by the subfield method, the dynamic resolution is greatly influenced by the array of light emitting weights for the respective subfields in each field. To prevent degradation of dynamic resolution, it is preferable to perform light emission, based on a video signal that arrives for one field, as impulses for a very short period within each field period. In the CRT display devices, one field period is required for horizontal and vertical scan processing, however, impulse-like light emission is made for one pixel at a particular display screen position, in each field.
However, in the gradation representation by the subfield method, as the video signal that arrives for one field is divided into a plurality of subfields within the field for light emission and display, impulse light emission cannot be made for a short period. For this reason, it is difficult to realize a dynamic resolution characteristic equivalent to that of the CRT device.
Hereinbelow, the phenomenon where the dynamic resolution is degraded in correspondence with the array of light emitting weights for subfields will be described with reference to FIG. 5, FIGS. 6A and 6B and FIGS. 7A and 7B. In this case, the white rectangular pattern 32 shown in FIG. 3 is displayed by a display device having a subfield arrangement for 64 (level xe2x80x9c0xe2x80x9d to level xe2x80x9c63xe2x80x9d) level representation with six subfields in FIG. 5. In a white (level xe2x80x9c63xe2x80x9d) pixel, light emission is performed in all the subfields SF0 to SF5 in one field, and the ratios of light emission intensities are 16:4:1:2:8:32. This means the array of light emitting weights is made such that energy concentrates at the head and the end of the field.
FIG. 6 shows a v-shaped angular light-emitting luminance distribution in a case where light emitting weights for the subfields are arranged such that the light emitting weight gradually decreases and then gradually increases in each of field 1, field 2, . . . of sequentially inputted video signals. In this v-shaped light emission type subfield arrangement, light emission most highly concentrates around a boundary T1 between fields, and intense light emission occurs at field periods. In the boundary T1, light emission in the first field and that in the second field mix with each other. When the moving rectangular pattern is displayed, two images overlap with each other with a time lag therebetween as represented with the solid line in FIG. 7A. Thus, an image with seriously degraded resolution is perceived.
For example, if light emitting response time of the G-cell is slow, a pattern represented with the broken line in FIG. 7A is detected. Similar to FIGS. 4A and 4B, in edge areas A1 and A2, a color of magenta is perceived due to shortage of amplitude of G light emission, and in edge areas B1 and B2, a color of green is perceived due to excess amplitude of G light emission.
In this case, as the two images overlap with each other with a time lag therebetween, the resolution is degraded, and the luminance does not change abruptly. Accordingly, in comparison with the color fringing in FIGS. 4A and 4B, the range of interference is wider, while the density of false colors (magenta and green) is lower. In this manner, the arrangement of light emitting weights for the subfields and the response characteristics of the R, G and B cells are closely related with each other. As the arrangement of light emitting weights for the subfields reduces color fringing interference at edge portions due to the differences in light emitting response characteristics of the R, G and B cells, both characteristics must be optimized so as to realize high-quality moving image reproduction.
Note that the gradation representation by using the subfield method is disclosed in Japanese Examined Patent Publication No. 51-32051, for example, and a method to reduce false contour noise characteristic of the subfield method is disclosed in Japanese Examined Patent Publication No. 4-211294, for example.
In the above-described conventional color image display devices, regarding the light emitting response characteristics of R, G and B cells, the image quality of a still image is treated as first priority. In those devices, fluorescent materials are selected in consideration of chromaticity coordinates, white balance conditions and luminous efficiency and the like, however, light emitting response characteristics based on the image quality of a moving image have not been considered, otherwise, even if considered, the light emitting response characteristics of the respective cells are shortened as much as possible only to reduce persistence.
Further, in the subfield method, the array of light emitting weights for subfields is determined only to reduce flicker or false contour interference, characteristic of this method, however, the degradation of dynamic resolution characteristic has not been considered.
Further, in the conventional color image display devices, the interaction between the light emitting response characteristics of R, G and B cells and the array of light emitting weights for subfields has not been considered.
Accordingly, in the above-described conventional color image display devices, when a moving image is displayed, R, G and B light emission timings shift from each other due to the differences in light emitting response characteristics of R, G and B cells. Therefore, a color not included in the original image is perceived at an edge portion, and the image quality is seriously degraded.
Further, even in a case where the light emitting response characteristics of R, G and B cells are increased, if the arrangement of light emitting weights for subfields is inappropriate, the dynamic resolution characteristic cannot be improved.
Generally, when one field is divided into M subfields, and light emitting weights corresponding to powers of 2 are allotted to the subfields, gradation representation can be made to the maximum level 2M. However, if light emitting weights which are not powers of 2 are allotted to the subfields or the subfields are divided so as to perform processing to remove false contour, characteristic of the subfield method, the number L of display gray scale levels for each pixel, with respect to the number M of the subfields, is less than 2M. That is, the number of subfields increases to realize the same display gray scale level. In this manner, when the number of subfields has increased, light emission is dispersedly performed within one field, which degrades the dynamic resolution.
Accordingly, an object of the present invention is to solve the problems of the above-described conventional techniques and to provide a color image display apparatus with an excellent dynamic resolution characteristic, which displays a high-quality moving image where color fringes at moving image edge portions are inconspicuous. Another object of the present invention is to provide an image display apparatus which attains higher image quality by using the false-contour interference reducing method.
To attain the foregoing objects, the present invention provides the following constructions:
(1) The time response characteristics of light emission by red, green and blue light emitting cells correspond to respective red, green and blue colors.
This construction provides a color image display apparatus which displays a high-quality moving image where color fringes at moving image edge portions are inconspicuous.
(2) Assuming that the time response characteristics of light emission by red, green and blue light emitting cells have values TR, TG and TB, the difference between the values TR and TG is sufficiently less than that between the values TR and TB and that between the values TG and TB.
This construction reduces the degradation of image quality due to color fringing and enables high-quality moving image display, since color fringing occurs in an inconspicuous color of blue or yellow of low spectral luminous efficacy at moving image edge portions.
(3) Light emitting weights allotted to respective subfields are arranged such that the light emitting weight increases from the head and the end of the light emitting weight array toward the center.
This construction substantially concentrates light emission in a short period, which reduces the degradation of the resolution in moving image display, and enables high-quality moving image display.
(4) Among a plurality of subfields, light emitting weights [N], [2xc2x7N], [3xc2x7N] . . . [(Kxe2x88x921)xc2x7N], [Kxc2x7N], [(kxe2x88x921)xc2x7N] . . . [2xc2x7N] and [N] (K, N: natural numbers) are allotted to 2xc2x7Kxe2x88x921 upper subfields.
This construction disperses xe2x80x9clight emission changeoverxe2x80x9d when the gray scale level continuously changes without concentrating the light emission changeover at a particular gray scale level, thus simultaneously enables acquisition of excellent dynamic resolution characteristic and reduction of false contour interference.
(5) Light emitting weights array for subfields are arranged such that light emitting luminance has two peaks in one field period, and time interval between the light emitting luminance peaks is xc2xd of the one field.
This construction increases a light-emission pattern repetitive period to a period substantially twice of a field frequency, thus reduces flicker interference and false contour interference.
(6) In addition to the construction (5), the persistence time of green and red light emitting cells is substantially xc2xd of the field frequency or longer than xc2xd of the field frequency.
This construction smoothes light emission by light emitting response characteristics of the light emitting cells, thus reduces false contour interference and displays a high-quality moving image.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same name or similar parts throughout the figures thereof.